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Posted by DavidS on October 08, 2003 at 21:24:55:
In Reply to: Re: Anybody got good ideas for flotation/webletters? posted by brw on August 13, 19103 at 13:00:56:
Like brw said, an empty sealed box will be more bouyant than the same box filled with foam because the foam weighs more than air... but all the bouyancy of the empty box will depend critically on the integrity of the walls.
Breach and vent the empty box, and its bouyancy can disappear in a heartbeat. Breach the foam box, and you've got bouyancy until the foam soaks through with water (maybe soon for a permeable open-cell foam, maybe never for a good closed-cell foam). The foam isn't there to provide flotation, it's there to keep water from taking it away.
Is it likely an impact will breach both bottom and bulkhead? Don't know, but consider: When you need any flotation at all to save your boat (or life), you've already had at least one estimate turn out wrong.
Two-pound-per-cubic-foot foam only detracts 3% of a volume's bouyancy, a pittance. Or look at it this way: can you build an empty bouyancy compartment with adequately unbreachable walls for less than two pounds per cubic foot more than the minimal confinement you might need with foam? A 1-foot cube of 1/4" ply has 6 square feet of walls and will weigh about five pounds. Is that box strong enough to be leakproof when you need it? A cubic foot of 2# foam wrapped in polyethylene and faced two sides with plywood for attachment would weigh about four pounds or so... providing the same bouyancy as the box with less dependence on leakproof walls.
So what if gasoline destroys Styrofoam? You could sheathe it lightly to keep gasoline off the foam, and mount it accessibly for inspection and replacement in "peacetime" if it does get fuel-damaged. It's not like it's gonna suddenly disappear while it's trying to float your swamped boat; then the part you're using is underwater where gas can't sit on it, and nothing that happens to any part above water will make it sink.
The one thing that concerns me about flotation foam is that it be truly closed-cell and impermeable to water (or small-pored and reliably not water-wettable, but wettability is a fickle thing to preserve and rely upon). Otherwise, even inconsequential contact with free water (not necessarily immersion) can let capillary forces imbibe it full of water that won't drain out even if you provide drain holes or remove any sheathing. It might dry out... water vapor has no surface tension with air to hold it in, but with only diffusion to drive it out it's gonna take time.
Sorry, engineer rambling above. To the original question: Many production boats use the 2-part pour-in flotation foam to completely fill the space between hull and deck/liner/sole... but they're not made of wood. If your foam fits tightly to your hull and/or sole (either closely fitted or poured in place) and ever contacts water (even bilgewater), capillary forces can make it wet and keep it from draining, and the close spacing means little ventilation and slow drying. If it were me, I'd leave a meaningful ventilation gap all around the foam except for a few places needed for attachment; I wouldn't use poured-in foam without some means of preserving those ventilation gaps. If you do use poured-in foam, be sure to provide an exit route for the expanding foam, and make sure the structure won't distort from the pressure it can exert as the mixture stiffens.
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